In a known link providing such interconnection, the line interconnecting the two stations is a pair of lines, i.e. it has two individual lines set up in parallel between the two stations. Each individual line is constituted by optical fiber sections connected together in series. In order to compensate for line losses, each terminal includes amplifier apparatus which, for each line, has an optical amplifier which is typically constituted by an optically-pumped erbium-doped silica fiber. In general, the word "amplifier" is used below to designate a light guide that is suitable for amplifying a light wave when said light wave to be amplified travels therealong simultaneously with a pumping wave.
For the same purpose of compensating losses, in-line amplification apparatuses can also be distributed along the length of the link. Each of them then also includes one such amplifier for each line.
It is known to use in such amplifier apparatus a common pump or a plurality of common pumps for supplying the two pumping waves necessary for the two amplifiers. Passive optical couplers of the type having two groups of two branches can then direct a fraction of the power of said pump or said pumps to each of the amplifiers without interfering with their respective connections to the two individual lines of the link.
Furthermore, such a link must be robust, i.e. it must in particular be made in such a manner that the operation of the transmitters and of the receivers is disturbed as little as possible and as rarely as possible by light waves not intended for them.
Such robustness would appear to be obtained to a reasonable degree in the above-mentioned known link.
Another essential characteristic of such a link is the quantity of information it is capable of conveying per second. A link may have a plurality of information channels implemented by spectrum, time, or other multiplexing on one or more lines, and this quantity needs to be distinguished from the data rate of each channel. On this topic, it has been found that in some known links of the kind described above and that are already in operation, that the maximum quantity of information that was specified at the time they were established and which they are capable of conveying now appears to be insufficient.
This quantity could be increased either by increasing the data rate of the terminals of each line, or by increasing the number of terminals included in the stations and similarly increasing the number of lines interconnecting the stations, and in both cases that would be expensive. If increasing the number of terminals, a major portion of the cost of obtaining such an increase in the quantity of information would lie in setting up new lines.